Frequency modulation detector circuit



06:. 31, 1944. F, B; STONE 2,361,664

. FREQUENCY MODULATION DETECTOR CIRCUIT Filed Aug. 22, 1942 r Z/T/L/Z/IT/O/V 15 C/fiCU/T 7 1 -L 4 I Z Z? 2! 3 f H 4 24 E 40/ [3 31 32 CAPACITY/I Iv 7 +5 file/1 Up 32 I l .E. /"=30/1/ T -12 T a I 050444706 lNyE NTOR fkgp 5. -fro/vs.

ATTORNEY Patented Oct. 3-1, 1944 FREQUENCY MODULATION DETECTOR CIRCUIT Fred B. Stone, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 22, 1942, Serial No. 455,716

(Cl. IVS-100.4)

12 Claims.

My present invention relates to a circuit for converting frequency variations into corresponding amplitude variations, and more particularly to a novel method of, and means for, reproducing audio energy by a frequency modulation process.

It has been proposed in the past, as for example by C. M. Sinnett in application Serial No. 459,375, filed Sept. 23, 1942, to reproduce sound records, recorded in the well known amplitude variation manner, by converting the mechanical displacements produced during the recording processes into a variable frequency deviation of a predetermined radio frequency carrier. The frequency modulated carrier energy is then subjected to a detection process for converting the frequency deviations into corresponding amplitude variations. One of the important advantages of this method is that a capacity pick-up device can be employed with the frequency-variable oscillator. This pick-up device is highly economical compared to the relatively expensive types of electrical pick-up devices employed in the usual and conventional method of converting the sound vibrations into audio signal energy.

One of the main objects of my present invention is to provide a method of converting the frequency-variable, high frequency energy into audio signal energy by means of a highly economical and compact frequency modulation detection device, the detection device essentially comprising a single tube whose electrodes perform the functions of producing the high frequency oscillations whose mean frequency is to be deviated, and conversion of the frequency-variable oscillations into corresponding amplitude variations which are then subjected to linear amplification preferably.

Another important object of this invention is to provide in combination with an audiosource generally, a tube of the triode-heptode type which functions as a combined high frequency oscillator and frequency modulation detector.

Another object of my invention is to provide a record reproducing system which uses a single tube envelope housing the electrodes of a triode and a heptode, the triode functioning to provide oscillations whose frequency will be varied in accordance with the variations in amplitude of the audio signals provided by a capacity pick-up, and the heptode device functioning to provide a frequency modulation detector for converting the frequency variable oscillations into the original audio signals.

Still other objects of the invention are generally to improve the efllcie'ncy and simplicity of record reproducers of the frequency modulation type, and more especially to provide a highly compact and simplified and economical form of the latter type of device.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be undertsood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit embodying the invention;

Fig. 2 illustrates the detection characteristic of the detector portion of the circuit;

Fig. 3 illustrates the-oscillator voltage amplitude at the heptode inner control grid as the oscillator frequency is varied with respect to the frequency to which circuit l8l9 is tuned.

Referring to Fig. 1, there is shown a tube 1 which consists of an envelope housing the electrodes of a triode and a heptode. For example, this type of tube may be of the 6J8G type or of the 787 type. Such a tube has a common cathode element 2 which is connected to ground through the usual biasing resistor 3, the latter being shunted by an audio frequency bypass condenser l since the cathode current contains audio components. control grid 5, the cathode 2 and the plate 6. The heptode section of the tube is. provided by a second section of cathode 2, a plate 1 and five sucessive grid electrodes arranged betweenthe cathode 2 and plate 'I. These grids are designated by numerals 8, 9, III, II and I2 respectively.

The grid electrode l2 acts as a suppressor grid, and is connected back to the cathode. The plate 'I and grids II and 9 are connected to a common plus B point on the direct current supply source of the receiver. The plate I is connected through the output load resistor l3 to the plus B terminal. The grids II and 9 are connected in common through the voltage reducing resistor 14 to the plus B terminal. The grid end of resistor I4 is by-passed to ground for audio frequenc currents by condenser l5.

At this point it is sufllcient to point out that there willbe developed across resistor IS the audio signal voltage, and the latter is transmitted to any desired audio-utilization circuit by the coupling condenser l6. Preferably linear audio ampliflcation is provided for the audio signals. The

The triode section is provided bymodulation frequencies.

I inner control grid I of the heptode section is connected by lead I! to the high alternating potential side of a resonant circuit which consists of coil l8 shunted by condenser 19, the lower end is electrostatically shielded by the positive grids II and 9, is capacity coupled to lead H. In tubes of the type used, and at frequencies of the order;

. of the circuit b g grounded. The grid l0, which of 18 mo. (megacycles) this capacity coupling of coupling. Grid 8'is tightly coupled, while grid Ill is coupled througha small value of capacitance. It is indicated on the drawin that the circuit l8l8 is tuned to a frequency F The frequency Fe is the mean,- or center, frequency of the oscillations produced by the triode section of tube I. These oscillations are provided in the following manner. The control grid 5 is connected toground through a path comprising lead 2l', condenser 22 and tank circuit coil 23. The grid end of condenser 22 is returned to ground through the'resistor 24.. The plate 6 is connected to the plus B terminal of the direct current supply source through a path comprising lead 30, the feedback coil 3| and resistor 32. 3i and 23 are reactively coupled so as. to provide regenerative feedback from the plate 6 to the tank, or grid, circuit of the triode oscillator. Th plate side of resistor 32 is bypassed to ground by condenser 32;

The coil 23 and condenser 22 are so chosen asto provide a tank circuit which is normally tuned to a frequency Fe. In the absence of applied modulation, this circuit produces oscillations of a mean, or center, frequency. The frequency of oscillations will be deviated in accordance with the amplitude of the applied modulation energy, and the rate of deviation will be a function of the This, of course, is the well-known frequency modulation process. Y From a genericviewpoint, I wish it clearly to be under stood that my invention-comprehends the utilization of phase modulation as well. Hence, I have used the general term angular velocity-modulated to designate that the modulation employed The coils 3 herein may be either frequency or phase modulation, and even hybrids thereof.

The modulation source is designated by numeral 4B. Merely by way of specific illustration the device 40 is shown as a capacity pick-up device. Such a capacity pick-up device is well known in phonograph recordreproduction, and generally comprises the spaced plates of a condenser. One plate of the condenser is connected to a stylus. The pointed end of the stylus is used I to ride through the sound track of a phonograph record. Hence, predetermined motion of the stylus producesa desired capacitance variation. Those skilled in the art are fully aware of the fact that in this way the capacity across the -.tive grids I0 and 8 in substantially phase quadrathe center frequency of the oscillator.

ture relation. A slight increase in coupling capacitance 20, as by an external condenser, results in some increase in performance.

I wish it to be clearly understood that the device 40 is merely illustrative. Any audio source of energy may be employed to vary the effective reactance of a reactance device across the coil 23. Again, the modulation source need not be in the audio range, but can even be sub-audible or super-audible. By means of the capacitance coupling between the inner and outer control grids 8 and II) respectively, the resultant modulation of the heptode plate current is made to vary with frequency thereby giving a range of operation over which the output plate current is a substantially linear function of frequency. The modulating capacitance 40 may beconnected across either the oscillator circuit or the-inner grid circuit l8l9. In the latter case a higher Q circuit can be realized due to the absence of oscillator grid loading. This may result in some increase in sensitivity.

A portion of the direct current flowing in the plate circuit connected to plate 1 of tube l is the direct current increment produced by the oscillator voltage applied to the outer control grid [0. The circuit l8l9, which is connected in series with the inner control grid 8, is tuned to the mean oscillator frequency Fe, and, therefore, it develops maximum oscillator voltage thereacross' through the capacitance coupling 20. However, the voltage across circuit I8--l9 is approximately in quadraturewith the voltage existing on the outer grid l0. As the frequency of the oscillations applied to grid [0 is varied, the amplitude and phase of the voltage across the circuit Ill-i9 changes. When the voltage of grid I0 is of a higher frequency than that to which circuit Iii-l9 is tuned,'the voltage across the latter approaches the same phase as the voltage of grid I0 thereby increasing the output plate current flowing through resistor l3. However, on the other side of the center frequency Fe, the voltage across circuit |8--l9 varies from degrees out of phase with the voltage of grid I0 through degrees out of phase with the latter voltage. Hence, the output plate current decreases through a minimum, and then approaches the no-signal value.

In Fig. 2 I have shown the variation of current flowing in the circuit connected to plate 1 as the frequency of the oscillations produced by the triode sections varies to and fro with respect to It will be understood that the curve has the typical characteristic which is required for a detector of frequency modulated carrier energy. While the frequency of oscillations may be of any desired value, I prefer to pick a value of 25 to 30 megacycles for the frequency Fe.. The range of free quency deviation of oscillations will extend up to a maximum of' about 15 kc. (kilocycles) on either side of Fe, where Feequals approximately 30 me. However, the frequency spacing between the peaks of the curve shown in Fig. 2 may be larger if desired. In Fig. 3 I have'shown graphically the manner in which the oscillator voltage amplitude varies on the heptode inner control grid as the oscillator frequency .is varied relative to the frequency Fe to which circuit l8-I9 is tuned. 1

The present circuit has several advantages. In the first place, an amplification effect is involved in the process which converts the frequency variations to amplitude variations. This is analogous to the conversion gain obtained by virtue of the conversion transconductance in normal frequency converter operation. It would appear, therefore, that less audio gain is required than is necessary with a separate oscillator tube followed by a diode detector. Secondly, the single tube system for both generating a frequency modulated signal and converting the frequency modulated signal into amplitude variations, corresponding to the rate and degree of frequency modulation, may be of advantage because of its compactness. As the compactness of the system is increased, it becomes more feasible to mount the single tube on the phonograph tone arm. This will result in a reduction of lead capacitance so that the percent modulation of the total capacitance is creased.

While I have indicated and described a system for carrying my invention into efiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but \that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with an adjustable capacity pick-up device whose capacitance is adapted to be varied by the sound track of a record, an oscillator tuned to a normal frequency, said capacity being connected to the oscillator to vary the frequency thereof in accordance with said capacitance variations, an electron discharge device provided with at least two spaced control electrodes in a common electrode stream, a resonant circuit, tuned to said normal frequency, connected to oneof the control electrodes, reactive means coupling said control electrodes,

means applying the frequency-variable oscillations to one of said control electrodes, and means for deriving from the space current path of said device signal amplitude variations corresponding to the record sound track. V g

2. In combination with an adjustable capacity whose capacitance is adapted to be varied by the sound track of a record, an oscillator tuned to a normal radio frequency, said capacity being connected to the oscillator to vary the frequency thereof in accordance with said capacitance variations, an electron discharge tube provided with at least a cathode, two spaced control e ectrodes and a plate, said two control electrodes being located in the electron stream from the cathode to said plate, a resonant circuit, tuned to said normal frequency, connected to one of the control electrodes, capacity means of small magnitude coupling said control electrodes, means applying the frequency-variable oscillations to one of said control electrodes, and means for deriving from the said plate signal amplitude variations corresponding to the record sound track.

3. In combination with an adjustable reactance pick-up device whose reactive value is adapted to be varied by the sound track of a record, an oscillator tuned to a normal frequency, said reactance being connected to the oscillator to vary the frequency thereof in accordance with said reactive variations, an electron discharge device provided with at least two spaced control electrodes in a common electron stream, a resonant the space current path of said device signal amplitude variations corresponding to the record sound track.

4. In combination with a source of audio signal energy, an oscillator comprising at least a cathode, control grid and anode, a resonant circuit, tuned to a predetermined radio frequency, coupled to said oscillator electrodes to provide oscillations of the predetermined frequency, means for varying the value of said frequency in response to said audio energy, a conversion circuit for converting the said frequency-variable oscillations into amplitude variations corresponding to said audio signals, said conversion circuit comprising an electron discharge device provided with atleast a cathode, anode and a pair of spaced control electrodes, a second resonant circuit tuned to said predetermined frequency, reactive means coupling said. spaced control electrodes, means connecting the coupled electrodes to the second resonant circuit, and means applying said frequency-variable oscillations to at least one of the spaced control electrodes.

5. In combination with a source of audio signal energy, an oscillator comprising at leasta cathode, control grid and anode, a resonant circuit, tuned toa predetermined radio frequency, coupled to said oscillator electrodes to provide oscillations of the predetermined frequency, condenser means for varying the value of said frequency in response to said audio energy, a conversion circuit for converting the said frequency-variable oscillations into amplitude variations corresponding to said audio signals, said conversion circuit comprising an electron discharge tube provided with at least a cathode, positive anode and a pair of spaced control grids, a second resonant circuit "tuned to said predetermined frequency, capacitance coupling said spaced control grids, means connecting the coupled grids to the second resonant circuit, and means applying said frequencyvariable oscillations to at least one of the spaced control grids.

6. In combination with a source of audio signal energy, an oscillator comprising at least a cathode, control grid and anode, a resonant circuit,

tuned to a predetermined radio frequency, coupled to said oscillator electrodes to provide oscillations of the predetermined frequency, means for varying the value of said frequency in response to said audio energy, a conversion circuit for converting the said frequency-variable oscillations into amplitude variations corresponding to said audio signals, said conversion circuit comprising an electron discharge device provided with at least a cathode, anode and a pair of spaced control electrodes, a common tube envelope housing the electrodes of said oscillator and conversion device, a second resonant circuit tuned to said predetermined frequency, reactive means coupling said spaced control electrodes, means connecting the coupled'electrodes to the second resonant circuit, and means applying said frequency-variable oscillations to at least one of the spaced control electrodes.

7. In combination with a phonograph record stylus, an oscillator comprising at least a cathode, control grid and anode, a resonant circuit,

circuit, tuned to said normal frequency, connected I to one of the control electrodes, capacity means coupling said control electrodes, means applying the frequency-variable oscillations to one of said control electrodes, and means for deriving from piitude variations corresponding to said audio signals, said conversion circuit comprising an electron discharge device provided with at least a cathode, anode and a pair of spaced control electrodes, a common tube envelope housing the electrodes of said oscillator and conversion device, a second resonant circuit tuned to said predetermined frequency, capacitance coupling said spaced control electrodes, means connecting the coupled electrodes to the second resonant circuit, and means applying said frequency-variable oscillations to at least one of the spaced control electrodes.

8. In combination with a single tube provided with an oscillator section and a detector section, said detector section comprising at least two spaced control grids, a first resonant circuit, tuned to a predetermined high frequency, coupled to the oscillation section, a second resonant circuit, tuned to said predetermined frequency, connected to one of said spaced grids, means connecting the first resonant circuit to the second of the spaced grids, capacitance coupling said "vice provided with atileast two spaced control electrodes in a common electron stream, a resonant circuit, tuned to said normal frequency, connected to one of the control electrodes, reactive means of predetermined magnitude coupling said control electrodes, means connecting said pickup device to at least one of said oscillator and resonant circuits therebyto vary the frequency thereof, means applying the resulting frequency-variable current to one of said control electrodes, and means for deriving from the space current path of said device signal amplitude variations corresponding to the record sound track.

11. In a system of the type comprising an oscillator section and a detector section, said detector section comprising at least two spaced a control grids in a common electron stream, a

spaced control grids thereby establishing a quad rature phase relation between the voltages on the said spaced control grids at said predetermined frequency, and means for varying the frequency of one of said two resonant circuits thereby to vary said quadrature phase relation whereby there is produced a substantiallylinear variation in detector output current.

9. In combination with a single tube provided with an oscillator triode section and a conversion heptode section, said heptode section comprising spaced inner and outer control grids, a first resonant circuit, tuned to a predetermined high frequency, coupled to the oscillator section, a second resonant circuit, tuned to said predetermined frequency, connected to the inner of said spaced grids, means connecting the first resonant circuit to the outer of the spaced grids, capacitance means coupling said spaced control grids thereby establishing a quadrature phase relation between the voltages on the said spacedcontrol grids at 7 first resonant circuit, tuned to a predetermined high frequency, coupled to the oscillation section, a second resonant circuit, tuned to said predetermined frequency, connected to one of said spaced grids; the improvement comprising means connecting the first resonant circuit to the second of the spaced grids, means providing sufilcient capacitance coupling between said spaced control grids to establish a substantial quadrature phase relation between the voltages on the said spaced control grids at said predetermined frequency,

and means for varying the frequency of one of said two resonant circuits thereby to vary said quadrature phase relation.

12. In a system of the type comprising a single tank circuit to the outer one of the spaced grids,

inter-electrode capacitance acting as means coupling said spaced control grids thereby establishing a substantial quadrature phase .relation between the voltages on the said spaced control grids at said predetermined frequency, and signal-responsiive reactive means for varying the frequency of the resonant tank circuit thereby to vary said quadrature phase relation and producing a signal variation in heptode'output current.

FRED B. STONE. 

